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In biological terms, therelative fitness of an organism is determined by the number of offspring produced by an organism in its lifetime.In other words, the maximum reproductive rate of the organism is relative to its absolute fitness.

For example, if an organism has the capacity to reproduce three offspring in its lifetime but it produces four offspring, the organism has higher relative fitness.

Mules are hard-working animals created from the cross between horse and donkey. They are sterile due to the presence of uneven chromosomes.Horses have 64 chromosomes, and donkeys have 62.

The cross between these two organisms gives mules (63 chromosomes). The uneven proportions of donkey and horse chromosomes fail to pair up in mules, which makes them sterile.

Mules are healthy and working animals. They also show a long lifespan. Although, they are sterile due to their chromosomal arrangement.Therefore, an organism that has a good life span but cannot give birth to young ones has a relative fitness of zero.

However, an organism with a shorter life span but of sterile nature could have high relative fitness.

The mule does not contribute to the next generation due to its sterility. Thus, it has zero relative fitness.